I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without batting
an eyelid).

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel. But there
was plenty of fuel. And the fuel is about 1 month old - it's not been
standing around for ages for its most volatile fractions to evaporate.

I found several things that *could* have been the cause - but weren't. There
was a build-up of grass mowings stuck to the underside of the mower, which
could have been making the blade harder to turn - fixed that but the problem
persisted. The air filter could have been blocked - the sponge filter looked
clean but was dripping in oil, so I cleaned it in hot soapy water and gave
it a *light* coating of oil as the manual says, but to no avail. The
sparking plug was coated in a thin layer of black carbon, but the gap looked
OK and there was no coking-up of the contacts - I cleaned the carbon off the
underside of the tag and the end of the rod (ie the surfaces between which
the spark jumps) but that didn't help either.

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly. My impression is that it seems to be using
a bit more fuel and that the smell of the exhaust is stronger, so could the
mixture be richer? The regulation of speed (ie the governor) seems to be as
good as ever: when the mower encounters tougher grass, the engine note gets
louder but not slower. I've tried with the drive to the wheels disengaged
(ie I have to push the mower) in case that was a factor - but it made no
different.

Any ideas? Is it time to "get it looked at"?

"NY" Wrote in message:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without batting
an eyelid).

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel. But there
was plenty of fuel. And the fuel is about 1 month old - it's not been
standing around for ages for its most volatile fractions to evaporate.

I found several things that *could* have been the cause - but weren't. There
was a build-up of grass mowings stuck to the underside of the mower, which
could have been making the blade harder to turn - fixed that but the problem
persisted. The air filter could have been blocked - the sponge filter looked
clean but was dripping in oil, so I cleaned it in hot soapy water and gave
it a *light* coating of oil as the manual says, but to no avail. The
sparking plug was coated in a thin layer of black carbon, but the gap looked
OK and there was no coking-up of the contacts - I cleaned the carbon off the
underside of the tag and the end of the rod (ie the surfaces between which
the spark jumps) but that didn't help either.

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly. My impression is that it seems to be using
a bit more fuel and that the smell of the exhaust is stronger, so could the
mixture be richer? The regulation of speed (ie the governor) seems to be as
good as ever: when the mower encounters tougher grass, the engine note gets
louder but not slower. I've tried with the drive to the wheels disengaged
(ie I have to push the mower) in case that was a factor - but it made no
different.

Any ideas? Is it time to "get it looked at"?



Carb service
--
Jimk


----Android NewsGroup Reader----
http://usenet.sinaapp.com/

"Jimk" wrote in message
o.uk...
"NY" Wrote in message:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering
and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut
a
trivial amount of grass (paradoxically, it cuts longer grass without
batting
an eyelid).
....
Any ideas? Is it time to "get it looked at"?

Carb service

Ah, do carbs need servicing from time to time? I don't remember my older
cars (those which had carbs) having an explicit task of servicing the
carburettor. I wasn't sure whether modern mowers still actually used a
carburettor as opposed to fuel injection.

Oh well, time to find someone who services mowers. At least I've eliminated
things that I could fix myself...

On 11/07/2020 20:53, NY wrote:
After a
year's perfect service, the engine has suddenly started coughing,

Is this a dry cough?

NY wrote

I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without
batting an eyelid).

Bet that last detail is crucial. Likely it has an automatic load control
that applys more power when the engine slows down and that
has failed so that it only works properly with a significant extra
load and isnt getting enough fuel on a light load.

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel.

And that is crucial info too. Something is producing
that situation on a light load but not on a heavier load.
Almost certainly the speed control given that it cant
be fuel starvation on the heavier load.

But there was plenty of fuel. And the fuel is about 1 month old - it's not
been standing around for ages for its most volatile fractions to
evaporate.

And wouldn’t produce that effect at different loads.

I found several things that *could* have been the cause - but weren't.
There was a build-up of grass mowings stuck to the underside of the mower,
which could have been making the blade harder to turn -

But that shouldn’t see it work fine on the longer grass.

fixed that but the problem persisted.

Unsurprisingly.

The air filter could have been blocked

Again, that should see it not work properly
on the longer grass that needs more air.

- the sponge filter looked clean but was dripping in oil, so I cleaned it
in hot soapy water and gave it a *light* coating of oil as the manual
says, but to no avail.

Unsurprisingly.

The sparking plug was coated in a thin layer of black carbon, but the gap
looked OK and there was no coking-up of the contacts

Again, unlikely to see the problem only with normal grass.

I cleaned the carbon off the underside of the tag and the end of the rod
(ie the surfaces between which the spark jumps) but that didn't help
either.

Unsurprisingly.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning,

Presumably because there is no fuel getting thru.

usually when cutting short sparse grass, rather than when it's working
harder to cut denser or longer grass.

See at the top.

And once it starts stalling, it does it repeatedly. My impression is that
it seems to be using a bit more fuel and that the smell of the exhaust is
stronger, so could the mixture be richer? The regulation of speed (ie the
governor) seems to be as good as ever:

But clearly isnt delivering enough fuel with normal grass.

when the mower encounters tougher grass, the engine note gets louder but
not slower.

I've tried with the drive to the wheels disengaged (ie I have to push the
mower) in case that was a factor - but it made no different.

Unsurprisingly.

Any ideas?

The problem is in the speed control. When it should be
delivering not as much fuel it is delivering very little or none.

Is it time to "get it looked at"?

If you arent confident of seeing what is wrong with the speed control.
They arent the easiest things to work on when they have failed.

NY wrote

I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without
batting an eyelid).

Bet that last detail is crucial. Likely it has an automatic load control
that applys more power when the engine slows down and that
has failed so that it only works properly with a significant extra
load and isnt getting enough fuel on a light load.

Either the speed control isnt working properly with normal grass
or the flow of fuel isnt happening properly just at the flow rate
needed for normal grass but does at lower or higher flow rates.
That last is less likely given how fiddly speed controls are.

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel.

And that is crucial info too. Something is producing
that situation on a light load but not on a heavier load.
Almost certainly the speed control given that it cant
be fuel starvation on the heavier load.

But there was plenty of fuel. And the fuel is about 1 month old - it's not
been standing around for ages for its most volatile fractions to
evaporate.

And wouldn’t produce that effect at different loads.

I found several things that *could* have been the cause - but weren't.
There was a build-up of grass mowings stuck to the underside of the mower,
which could have been making the blade harder to turn -

But that shouldn’t see it work fine on the longer grass.

fixed that but the problem persisted.

Unsurprisingly.

The air filter could have been blocked

Again, that should see it not work properly
on the longer grass that needs more air.

- the sponge filter looked clean but was dripping in oil, so I cleaned it
in hot soapy water and gave it a *light* coating of oil as the manual
says, but to no avail.

Unsurprisingly.

The sparking plug was coated in a thin layer of black carbon, but the gap
looked OK and there was no coking-up of the contacts

Again, unlikely to see the problem only with normal grass.

I cleaned the carbon off the underside of the tag and the end of the rod
(ie the surfaces between which the spark jumps) but that didn't help
either.

Unsurprisingly.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning,

Presumably because there is no fuel getting thru.

usually when cutting short sparse grass, rather than when it's working
harder to cut denser or longer grass.

See at the top.

And once it starts stalling, it does it repeatedly. My impression is that
it seems to be using a bit more fuel and that the smell of the exhaust is
stronger, so could the mixture be richer? The regulation of speed (ie the
governor) seems to be as good as ever:

But clearly isnt delivering enough fuel with normal grass.

when the mower encounters tougher grass, the engine note gets louder but
not slower.

I've tried with the drive to the wheels disengaged (ie I have to push the
mower) in case that was a factor - but it made no different.

Unsurprisingly.

Any ideas?

The problem is in the speed control. When it should be
delivering not as much fuel it is delivering very little or none.

Is it time to "get it looked at"?

If you arent confident of seeing what is wrong with the speed control.
They arent the easiest things to work on when they have failed.

On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:

snip

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly.

Valve gaps? As the engine heats up and stuff expands the valve gaps
get smaller, the engine heats up on the longer bits and so appears to
misbehave when on the shorter stuff? shrug

Take the rocker cover off (often easy on such things) when it's hot
and check you still have play on both valves (you make have to turn
the engine over a bit to check both valves (make sure it's off, pull
the plug cap off, keep hands away from everything etc)).

You often need around 10 thou gap (they can be different). Easy to
check / adjust yourself but you will need a feeler gauge set (cheap)
and a spanner / screwdriver.

Cheers, T i m

"NY" wrote in message
...
"Jimk" wrote in message
o.uk...
"NY" Wrote in message:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a
year's
perfect service, the engine has suddenly started coughing, spluttering
and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut
a
trivial amount of grass (paradoxically, it cuts longer grass without
batting
an eyelid).
...
Any ideas? Is it time to "get it looked at"?

Carb service

Ah, do carbs need servicing from time to time?

Nope, not if you keep using clean fuel and don't
leave it over winter with fuel in the tank, let it run
dry with the last mowing in autumn.

I don't remember my older cars (those which had carbs) having an explicit
task of servicing the carburettor.

Cars are different because they wont evaporate old fuel
when standing around for months unused and you don't
usually not use them all winter etc.

I wasn't sure whether modern mowers still actually used a carburettor as
opposed to fuel injection.

Few do use fuel injection.

Oh well, time to find someone who services mowers. At least I've
eliminated things that I could fix myself...

But would have done better to consider what could produce the symptoms you
were seeing.

"Chris Hogg" wrote in message
...
On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:


Any ideas? Is it time to "get it looked at"?

Dirt in the fuel tank blocking a filter?

That wouldn't see it do the long grass fine.

There's a video on cleaning
the carb on your B&S 125cc 450e engine, here
https://www.youtube.com/watch?v=v_4MurQyBgU

--

Chris

On Sat, 11 Jul 2020 23:49:20 +0100, T i m wrote:

On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:

snip

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly.

Valve gaps? As the engine heats up and stuff expands the valve gaps
get smaller, the engine heats up on the longer bits and so appears to
misbehave when on the shorter stuff? shrug

Take the rocker cover off (often easy on such things) when it's hot
and check you still have play on both valves (you make have to turn
the engine over a bit to check both valves (make sure it's off, pull
the plug cap off, keep hands away from everything etc)).

You often need around 10 thou gap (they can be different). Easy to
check / adjust yourself but you will need a feeler gauge set (cheap)
and a spanner / screwdriver.

This gives you and idea of the sort of thing you might need to do (and
he explains the background a bit as well).

https://www.youtube.com/watch?v=U6NdqHHldUM

Yours could be completely different in the details (self locking nuts,
different gaps etc) but the principals / processes will be very
similar.

Even if the gaps turn out to be right (typically set cold), at least
you can them rule them out. ;-)

Cheers, T i m

On 11/07/2020 23:49:20, T i m wrote:
On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:

snip

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly.

Valve gaps? As the engine heats up and stuff expands the valve gaps
get smaller, the engine heats up on the longer bits and so appears to
misbehave when on the shorter stuff? shrug

In which case the engine is more likely to stall under heavy load; where
the valve stems will expand more with temperature.

The OP said "will suddenly stall without warning, usually when cutting
short sparse grass, rather than when it's working harder to cut denser
or longer grass."

On 11/07/2020 20:53:08, NY wrote:

The sparking plug was coated in a thin layer of
black carbon, but the gap looked OK and there was no coking-up of the
contacts - I cleaned the carbon off the underside of the tag and the end
of the rod (ie the surfaces between which the spark jumps) but that
didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

I would be tempted to follow the Youtube video for servicing the
carburettor that has been posted on this thread.

Fredxx wrote
NY wrote

The sparking plug was coated in a thin layer of black carbon, but the gap
looked OK and there was no coking-up of the contacts - I cleaned the
carbon off the underside of the tag and the end of the rod (ie the
surfaces between which the spark jumps) but that didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

But that wouldn’t vary with the load on the
engine due to the type of grass being cut.

That has to be the speed control misbehaving on a light load.

Does it tick over / idle without misfiring?

I would be tempted to follow the Youtube video for servicing the
carburettor that has been posted on this thread.

I’d check the speed control first. Bet that’s the problem.

Not as easy to check that tho.

NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without batting
an eyelid).

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel. But there
was plenty of fuel. And the fuel is about 1 month old - it's not been
standing around for ages for its most volatile fractions to evaporate.

I found several things that *could* have been the cause - but weren't. There
was a build-up of grass mowings stuck to the underside of the mower, which
could have been making the blade harder to turn - fixed that but the problem
persisted. The air filter could have been blocked - the sponge filter looked
clean but was dripping in oil, so I cleaned it in hot soapy water and gave
it a *light* coating of oil as the manual says, but to no avail. The
sparking plug was coated in a thin layer of black carbon, but the gap looked
OK and there was no coking-up of the contacts - I cleaned the carbon off the
underside of the tag and the end of the rod (ie the surfaces between which
the spark jumps) but that didn't help either.

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly. My impression is that it seems to be using
a bit more fuel and that the smell of the exhaust is stronger, so could the
mixture be richer? The regulation of speed (ie the governor) seems to be as
good as ever: when the mower encounters tougher grass, the engine note gets
louder but not slower. I've tried with the drive to the wheels disengaged
(ie I have to push the mower) in case that was a factor - but it made no
different.

Any ideas? Is it time to "get it looked at"?



Odd one. Could you have hit a large rock back a the start of all this? Im
wondering if the ignition timing is now out due to the flywheel having
slipped on the crankshaft. Its not unknown for the woodruff key to shear.

Tim

--
Please don't feed the trolls

On 11/07/2020 20:53, NY wrote:
Any ideas? Is it time to "get it looked at"?

dirt in the low speed jet.



--
Karl Marx said religion is the opium of the people.
But Marxism is the crack cocaine.

On Sun, 12 Jul 2020 00:50:48 +0100, Fredxx wrote:

On 11/07/2020 23:49:20, T i m wrote:
On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:

snip

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly.

Valve gaps? As the engine heats up and stuff expands the valve gaps
get smaller, the engine heats up on the longer bits and so appears to
misbehave when on the shorter stuff? shrug

In which case the engine is more likely to stall under heavy load; where
the valve stems will expand more with temperature.

How fast do you think metals expand under such circumstances?

The OP said "will suddenly stall without warning, usually when cutting
short sparse grass, rather than when it's working harder to cut denser
or longer grass."

I know and had already accounted for that (see above).

In any case, it might not be a bad idea to check the valve gaps eh?

Cheers, T i m

On Saturday, 11 July 2020 21:02:36 UTC+1, NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a
trivial amount of grass (paradoxically, it cuts longer grass without batting
an eyelid).

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel. But there
was plenty of fuel. And the fuel is about 1 month old - it's not been
standing around for ages for its most volatile fractions to evaporate.

I found several things that *could* have been the cause - but weren't. There
was a build-up of grass mowings stuck to the underside of the mower, which
could have been making the blade harder to turn - fixed that but the problem
persisted. The air filter could have been blocked - the sponge filter looked
clean but was dripping in oil, so I cleaned it in hot soapy water and gave
it a *light* coating of oil as the manual says, but to no avail. The
sparking plug was coated in a thin layer of black carbon, but the gap looked
OK and there was no coking-up of the contacts - I cleaned the carbon off the
underside of the tag and the end of the rod (ie the surfaces between which
the spark jumps) but that didn't help either.

I've got it to a state where the mower will usually run for tens of minutes,
with irregular misfiring and occasional backfires, but will suddenly stall
without warning, usually when cutting short sparse grass, rather than when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly. My impression is that it seems to be using
a bit more fuel and that the smell of the exhaust is stronger, so could the
mixture be richer? The regulation of speed (ie the governor) seems to be as
good as ever: when the mower encounters tougher grass, the engine note gets
louder but not slower. I've tried with the drive to the wheels disengaged
(ie I have to push the mower) in case that was a factor - but it made no
different.

Any ideas? Is it time to "get it looked at"?

Could be muck in the fuel tank blocking the outlet.
Or water in the fuel.
Or aprblem with the valves (do compression check).

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"harry" wrote in message
...
On Saturday, 11 July 2020 21:02:36 UTC+1, NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a year's
perfect service, the engine has suddenly started coughing, spluttering
and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut
a
trivial amount of grass (paradoxically, it cuts longer grass without
batting
an eyelid).

At first I thought it was running low on fuel, because the uneven running
sounds identical to that which leads up to running out of fuel. But there
was plenty of fuel. And the fuel is about 1 month old - it's not been
standing around for ages for its most volatile fractions to evaporate.

I found several things that *could* have been the cause - but weren't.
There
was a build-up of grass mowings stuck to the underside of the mower,
which
could have been making the blade harder to turn - fixed that but the
problem
persisted. The air filter could have been blocked - the sponge filter
looked
clean but was dripping in oil, so I cleaned it in hot soapy water and
gave
it a *light* coating of oil as the manual says, but to no avail. The
sparking plug was coated in a thin layer of black carbon, but the gap
looked
OK and there was no coking-up of the contacts - I cleaned the carbon off
the
underside of the tag and the end of the rod (ie the surfaces between
which
the spark jumps) but that didn't help either.

I've got it to a state where the mower will usually run for tens of
minutes,
with irregular misfiring and occasional backfires, but will suddenly
stall
without warning, usually when cutting short sparse grass, rather than
when
it's working harder to cut denser or longer grass. And once it starts
stalling, it does it repeatedly. My impression is that it seems to be
using
a bit more fuel and that the smell of the exhaust is stronger, so could
the
mixture be richer? The regulation of speed (ie the governor) seems to be
as
good as ever: when the mower encounters tougher grass, the engine note
gets
louder but not slower. I've tried with the drive to the wheels disengaged
(ie I have to push the mower) in case that was a factor - but it made no
different.

Any ideas? Is it time to "get it looked at"?

Could be muck in the fuel tank blocking the outlet.

That would see it do the normal grass fine and
not the long grass which needs more fuel.

Or water in the fuel.

Ditto.

Or aprblem with the valves (do compression check).

Ditto.

On 11/07/2020 20:53, NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a
year's perfect service, the engine has suddenly started coughing,
spluttering and backfiring, and sometimes repeatedly stalling as soon as
it tries to cut a trivial amount of grass (paradoxically, it cuts longer
grass without batting an eyelid).

At first I thought it was running low on fuel, because the uneven
running sounds identical to that which leads up to running out of fuel.
But there was plenty of fuel. And the fuel is about 1 month old - it's
not been standing around for ages for its most volatile fractions to
evaporate.

I found several things that *could* have been the cause - but weren't.
There was a build-up of grass mowings stuck to the underside of the
mower, which could have been making the blade harder to turn - fixed
that but the problem persisted. The air filter could have been blocked -
the sponge filter looked clean but was dripping in oil, so I cleaned it
in hot soapy water and gave it a *light* coating of oil as the manual
says, but to no avail. The sparking plug was coated in a thin layer of
black carbon, but the gap looked OK and there was no coking-up of the
contacts - I cleaned the carbon off the underside of the tag and the end
of the rod (ie the surfaces between which the spark jumps) but that
didn't help either.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass. And
once it starts stalling, it does it repeatedly. My impression is that it
seems to be using a bit more fuel and that the smell of the exhaust is
stronger, so could the mixture be richer? The regulation of speed (ie
the governor) seems to be as good as ever: when the mower encounters
tougher grass, the engine note gets louder but not slower. I've tried
with the drive to the wheels disengaged (ie I have to push the mower) in
case that was a factor - but it made no different.

Any ideas? Is it time to "get it looked at"?

Sounds like over fuelling. Too much fuel at low loads.

This will be one of two things either the gasket for the primer has
failed or the float valve needle has stuck open.

My money is on float valve.

Mike

The Natural Philosopher presented the following explanation :
On 11/07/2020 20:53, NY wrote:
Any ideas? Is it time to "get it looked at"?

dirt in the low speed jet.

Back in my carburettor days of driving cars, a trick of mine to easily
clear jets, was....

To remove the air filter, get the engine revving hard- then slap a hand
over and blocking the air intake. It puts maximum intake suction on the
jets, to suck any dirt through. It didn't always work, usually it
would, but it's a quick fix.

On 11/07/2020 22:16, Chris Hogg wrote:
On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:


Any ideas? Is it time to "get it looked at"?

Dirt in the fuel tank blocking a filter? There's a video on cleaning
the carb on your B&S 125cc 450e engine, here
https://www.youtube.com/watch?v=v_4MurQyBgU


I would clean the carburetor, particularly the jets. New ones are
quite cheap on ebay.


--
Michael Chare

On Sun, 12 Jul 2020 18:59:25 +1000, cantankerous trolling geezer Rodent
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"Fredxx" wrote in message
...
On 11/07/2020 20:53:08, NY wrote:

The sparking plug was coated in a thin layer of black carbon, but the gap
looked OK and there was no coking-up of the contacts - I cleaned the
carbon off the underside of the tag and the end of the rod (ie the
surfaces between which the spark jumps) but that didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

The engine misfires under all loads, from idling (blade turning, but clear
of grass) up to heavy load. It's the intermittent stalling that seems to
happen a lot more frequently under light load (but not idling).

The suggestions about the carburettor seem very plausible - particularly to
the mixture being too rich, given the increased fuel usage and the stronger
smell of the exhaust (incomplete combustion?).

Would over-rich mixture (without an accompanying problem with the governor
at light loads) explain the symptoms?

On Sun, 12 Jul 2020 18:59:25 +1000, "Rod Speed"
wrote:

snip

Any ideas? Is it time to "get it looked at"?

Could be muck in the fuel tank blocking the outlet.

That would see it do the normal grass fine and
not the long grass which needs more fuel.

Or water in the fuel.

Ditto.

Or aprblem with the valves (do compression check).

Ditto.


Except, things like that can start to be a problem *once* the engine
has fully warmed up.

Further, our car would start (/restart) when hot or cold, not when
warm, but would run perfectly well at all temperatures, including
warm.

I did sort a motorcycle for someone that gained an airleak on the
induction side on one carb once warm, only after the local bike shop
had charged him to strip, clean and re-jet the carbs ...

A batch of 4/ strimmers were losing power when run hard for a long
time but not if only run hard between periods of ticking over. That
was down to the valve gaps closing up ...

Cheers, T i m

NY wrote:
"Fredxx" wrote in message
...
On 11/07/2020 20:53:08, NY wrote:

The sparking plug was coated in a thin layer of black carbon, but the gap
looked OK and there was no coking-up of the contacts - I cleaned the
carbon off the underside of the tag and the end of the rod (ie the
surfaces between which the spark jumps) but that didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

The engine misfires under all loads, from idling (blade turning, but clear
of grass) up to heavy load. It's the intermittent stalling that seems to
happen a lot more frequently under light load (but not idling).

The suggestions about the carburettor seem very plausible - particularly to
the mixture being too rich, given the increased fuel usage and the stronger
smell of the exhaust (incomplete combustion?).

Would over-rich mixture (without an accompanying problem with the governor
at light loads) explain the symptoms?


I would have thought that most carb problems would lead to under-fuelling
rather than over-fuelling. I think it would be worthwhile checking the
flywheel alignment on the crankshaft. Im not giving up on my mistiming
theory. ;-)

Tim

--
Please don't feed the trolls

On 11/07/2020 20:53, NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a
year's perfect service, the engine has suddenly started coughing,
spluttering and backfiring, and sometimes repeatedly stalling as soon as
it tries to cut a trivial amount of grass (paradoxically, it cuts longer
grass without batting an eyelid).

At first I thought it was running low on fuel, because the uneven
running sounds identical to that which leads up to running out of fuel.
But there was plenty of fuel. And the fuel is about 1 month old - it's
not been standing around for ages for its most volatile fractions to
evaporate.

I found several things that *could* have been the cause - but weren't.
There was a build-up of grass mowings stuck to the underside of the
mower, which could have been making the blade harder to turn - fixed
that but the problem persisted. The air filter could have been blocked -
the sponge filter looked clean but was dripping in oil, so I cleaned it
in hot soapy water and gave it a *light* coating of oil as the manual
says, but to no avail. The sparking plug was coated in a thin layer of
black carbon, but the gap looked OK and there was no coking-up of the
contacts - I cleaned the carbon off the underside of the tag and the end
of the rod (ie the surfaces between which the spark jumps) but that
didn't help either.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass. And
once it starts stalling, it does it repeatedly. My impression is that it
seems to be using a bit more fuel and that the smell of the exhaust is
stronger, so could the mixture be richer? The regulation of speed (ie
the governor) seems to be as good as ever: when the mower encounters
tougher grass, the engine note gets louder but not slower. I've tried
with the drive to the wheels disengaged (ie I have to push the mower) in
case that was a factor - but it made no different.

Any ideas? Is it time to "get it looked at"?

Jeeez - FFS get a decent cordless mower, just a quick blade sharp-up
now and again and that's it ....

On Sat, 11 Jul 2020 21:42:57 +0100, NY wrote:

Ah, do carbs need servicing from time to time?

Yes but in the case of mower carb it normally because a lump of crud
has got into the wrong place and needs cleaing out.

I wasn't sure whether modern mowers still actually used a carburettor as
opposed to fuel injection.

Oh no, these small petrol engines are basic. There's discussion up
thread about the valves and adjusting them. I don't think they have
such adjustment, the parts are made the "correct" size so on assembly
everything is gapped OK (ish).

Oh well, time to find someone who services mowers. At least I've
eliminated things that I could fix myself...

Pretty easy to remove and clean the carb. Just don't fiddle with any
of the jet adjustments or poke things through 'em. Backwash with
petrol to flush anything out.

--
Cheers
Dave.

"NY" wrote in message
...
"Fredxx" wrote in message
...
On 11/07/2020 20:53:08, NY wrote:

The sparking plug was coated in a thin layer of black carbon, but the
gap looked OK and there was no coking-up of the contacts - I cleaned the
carbon off the underside of the tag and the end of the rod (ie the
surfaces between which the spark jumps) but that didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

The engine misfires under all loads, from idling (blade turning, but clear
of grass) up to heavy load. It's the intermittent stalling that seems to
happen a lot more frequently under light load (but not idling).

That’s crucial information that you should have included in the original.

The suggestions about the carburettor seem very plausible - particularly
to the mixture being too rich, given the increased fuel usage and the
stronger smell of the exhaust (incomplete combustion?).

Would over-rich mixture (without an accompanying problem with the governor
at light loads) explain the symptoms?

That wouldn’t normally produce the stalling on light grass but not on long
grass.

The timing could be way out due to hitting something solid and breaking
the key that has the flywheel fixed on the crankshaft. But again, hard to
see
why that would see it stall only on light grass.

T i m wrote
Rod Speed wrote

Any ideas? Is it time to "get it looked at"?

Could be muck in the fuel tank blocking the outlet.

That would see it do the normal grass fine and
not the long grass which needs more fuel.

Or water in the fuel.

Ditto.

Or aprblem with the valves (do compression check).

Ditto.

Except, things like that can start to be a problem
*once* the engine has fully warmed up.

But he didn't say the problem is only seen when it has warmed up.

And has now added the crucial information that it misfires all the
time and only stalls when used on light grass, not on heavy grass.
Your possibilitys don't explain that behaviour.

Further, our car would start (/restart) when hot
or cold, not when warm, but would run perfectly
well at all temperatures, including warm.

Not relevant to his symptoms.

I did sort a motorcycle for someone that gained an airleak on
the induction side on one carb once warm, only after the local
bike shop had charged him to strip, clean and re-jet the carbs ...

A batch of 4/ strimmers were losing power when run hard for
a long time but not if only run hard between periods of ticking
over. That was down to the valve gaps closing up ...

Again, not relevant the symptoms he has.

With any fault finding, you have to consider if a particular
potential fault could produce the symptoms he is seeing.

"Tim+" wrote in message
...
NY wrote:
"Fredxx" wrote in message
...
On 11/07/2020 20:53:08, NY wrote:

The sparking plug was coated in a thin layer of black carbon, but the
gap
looked OK and there was no coking-up of the contacts - I cleaned the
carbon off the underside of the tag and the end of the rod (ie the
surfaces between which the spark jumps) but that didn't help either.

Sooted spark plug does suggest carburation issues.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse
grass,
rather than when it's working harder to cut denser or longer grass.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

The engine misfires under all loads, from idling (blade turning, but
clear
of grass) up to heavy load. It's the intermittent stalling that seems to
happen a lot more frequently under light load (but not idling).

The suggestions about the carburettor seem very plausible - particularly
to
the mixture being too rich, given the increased fuel usage and the
stronger
smell of the exhaust (incomplete combustion?).

Would over-rich mixture (without an accompanying problem with the
governor
at light loads) explain the symptoms?


I would have thought that most carb problems would lead to under-fuelling
rather than over-fuelling. I think it would be worthwhile checking the
flywheel alignment on the crankshaft. Im not giving up on my mistiming
theory. ;-)

Yeah, it does look likely now that he includes the crucial information
that it misfires all the time. Not clear why it produces a stall on little
grass tho but that may be a quirk of the timing being way off now
that you dont normally see because that fault isnt common at all.

On Mon, 13 Jul 2020 05:42:11 +1000, cantankerous trolling geezer Rodent
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On Mon, 13 Jul 2020 05:38:39 +1000, cantankerous trolling geezer Rodent
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On Sunday, 12 July 2020 21:23:00 UTC+1, Peeler wrote:
On Mon, 13 Jul 2020 05:38:39 +1000, cantankerous trolling geezer Rodent
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"Who or What is Rod Speed?

For all his faults he makes more sense than you do
Do something useful with your life. There are plenty of options.

On Mon, 13 Jul 2020 02:13:19 -0700 (PDT), , the
troll-feeding senile Google groper, blathered again:

On Sunday, 12 July 2020 21:23:00 UTC+1, Peeler wrote:
On Mon, 13 Jul 2020 05:38:39 +1000, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

FLUSH the trolling senile asshole's latest troll**** unread

"Who or What is Rod Speed?

For all his faults he makes more sense than you do
Do something useful with your life. There are plenty of options.

LOL Just your stupid senile gob, you really stupid senile Google groper!
tsk